Syria's government was accused on Wednesday of carrying out a new massacre in a small village near the central city of Hama, with an opposition group claiming 100 people, including many women and children, had been killed.

"We have 100 deaths in the village of al-Qubair, among them 20 women and 20 children," said Mohammed Sermini, spokesman for the Syrian National Council, who accused the regime of being behind the incident.

The news looked certain to fuel a bitter debate about the increasingly bloody Syrian crisis and to underline the limits of what a deeply divided international community can achieve.

The British-based Syrian Observatory for Human Rights (SOHR) said the massacre was carried out at a farm by pro-regime shabiha militiamen armed with guns and knives after regular troops had shelled the area.

Reports and images of the incident spread rapidly on Twitter and other social networks but were impossible to verify independently given the lack of media access to much of Syria.

Online pictures showed charred corpses lying amid rubble and a dead child who had apparently been shot in the face.

The Local Co-ordination Committees (LCC), an activist network, had earlier reported shelling in al-Qubair and neighbouring Maazarif, with "dozens martyred and wounded." The villages are about 12 miles from Hama.

Opposition activists said women and children were among the dead when al-Qubair came under heavy tank fire before shabiha fighters moved in on the ground and shot and stabbed dozens of people to death. The LCC counted 78 victims, 35 of whom were said to be from one family.

"Qubair was stormed with very heavy and random gunfire, houses were broken into and the residents were killed, some with knives," said a Hama-based source. "There are also burnt bodies."

Syrian state TV said troops had attacked "terrorists" – the phrase the official media uses for any opposition, armed or peaceful, to the regime of President Bashar al-Assad.

The news came as international efforts to deal with the Syrian crisis appeared to have stalled after Russia and China again made clear their opposition to any kind of outside intervention after 15 months of bloodshed, an estimated 15,000 dead and a de facto civil war. Both countries have vetoed UN security council resolutions designed to pressure Assad.

Attention has focused sharply on Syria since at least 108 people were killed in a two-day massacre that began on 25 May near Houla, most of them women and children who were summarily executed, according to the UN. The incidents have underscored the failure of a ceasefire brokered by the UN envoy Kofi Annan in March. Annan is to brief the UN security council on Thursday in New York.

Members of the 300-strong force of UN monitors in Syria are likely to investigate the al-Qubair incident.

"If their mission is to merely watch Syrian people die during breaches of the ceasefire, rather than helping in stopping any breaches by being physically on the ground, then we do not require their help," said the SOHR.

Earlier, ministers and officials from 15 countries and the EU agreed at a meeting hosted by Turkey to convene a new "co-ordination group" to provide support to the Syrian opposition, but left unclear what it might involve.

Annan is said to be hoping to stop a total collapse of his six-point plan for a truce and negotiated political solution. Diplomats say he will be reluctant to admit it has failed in the absence of any viable alternative. Western governments have shown no appetite for Libyan-style military intervention outside the UN though there are increasing calls in the US to arm the anti-Assad rebels of the Free Syrian Army. Saudi Arabia and its Gulf allies are thought to be already doing so.